Such a vehicle is designed in particular for inspecting rigid pipes transporting hydrocarbons. The pipes are usually located at the seabed. The aim of the inspection is to detect potential defects resulting from corrosion of the pipes.
Traditionally, pipeline corrosion inspection is carried out with a diver and/or with a remotely operated vehicle (or “ROV”). It is therefore necessary to provide a surface facility above the pipes to be inspected. In the case of a diver, limitations exist regarding the depth at which the intervention can be done. ROV can operate at deeper depths. However, the surface installation is still needed. A link between the ROV and the surface assembly must be deployed to power the ROV. The link is for example an umbilical.
In a known measurement method, the ROV is controlled from the surface to follow the subsea structure. A deployable articulated arm extends from a side of the ROV to place a first proximity cathodic protection probe very close to the pipe to be sensed.
A second remote cathodic protection probe is immersed into the body of water from the surface facility, at a distance from the surface facility to obtain a reference measurement outside the electrical field.
A voltage difference between the proximity probe and the remote probe is measured along the pipe. From time to time, the ROV is touching the pipe directly (at an anode connection for instance), in order to calibrate the voltage difference between the electrode close to the pipe and the one in shallow water.
The method allows a precise corrosion measurement of the surface of the pipe. Nevertheless, the method is time consuming, expensive and requires complex equipment such as a ROV and an associated surface assembly connected to the ROV.
To partially alleviate this problem, US 2014/0230713 discloses an underwater mobile inspection apparatus which is able to cruise autonomously above a pipeline. The apparatus comprises an articulated arm equipped with a proximity cathodic protection probe. The articulated arm is attached to a side of the vehicle.
Such a device is not entirely satisfactory. Indeed, the proximity cathodic protection probe is carried on a cart which rolls on the pipeline. The cart is deployed from the articulated arm, creating a strong drag when cruising the inspection apparatus above the pipe and generates high friction on the pipeline.
As a consequence, the inspection apparatus can only be operated at very slow speeds, with a substantial risk of damaging the surface of the pipeline.
One aim of the invention is to obtain an underwater vehicle suitable for corrosion inspection of a subsea structure, the vehicle being able to be operated at a high speed above the subsea structure.